PHONON-SCATTERING IN CHEMICAL-VAPOR-DEPOSITED DIAMOND

The in-plane thermal conductivity kappa(parallel-to) has been measured over the temperature range 5-400 K for samples of chemical-vapor-deposited (CVD) diamond made by both the microwave and hot-filament processes. The samples span a range of defect level, grain size, and degree of thinning. Comparison with a model of heat transport suggests that kappa(parallel-to) is limited by scattering of phonons from point defects, extended defects of approximately 1.5 nm diameter, dislocations, grain boundaries, and microcracks, as well as by phonon-phonon scattering at high temperatures. The Callaway model of thermal conductivity is used to include the effects of normal three-phonon scattering processes. In the higher-conductivity samples, scattering of long-wavelength phonons is very weak even at grain boundaries, indicating relatively smooth boundaries. The value of kappa(parallel-to) = 20 W cm-1 K-1 at room temperature for some of the microwave CVD samples is the highest reported to date for CVD diamond. Measurements of the anisotropy in conductivity obtained from the measured perpendicular conductivity kappa(perpendicular-to) consistently show a higher conductivity along the (columnar) grains. The hot-filament-CVD sample measured exhibits a room-temperature conductivity approaching that of the best microwave-plasma samples, indicating that the thermal conductivity is determined more by the specific conditions of growth than by the type of CVD growth (microwave or hot filament).